Such a method is known from U.S. Pat. No. 4,891,837. The compander compresses the signals produced by the microphone, hereafter called transmitted signals, to a uniform signal level insofar as they are above a nominal value, and amplifies them to the maximum insofar as they are at the nominal value, and expands them insofar as they are below the nominal value. The compander curve LU2=f(LU1) has a steep expansion range and a flat compression range. The curve can be displaced or the steepness of the curve can be changed in the expansion range by feeding the output signals back to the input of the compander, through a low pass or a band-pass filter. In addition, the curve displacement can be produced in the receiving branch as a function of volume control. As a rule, the system parameters are adjusted by the manufacturer to the normal conditions of an office environment. A manual adjustment must be made if the conditions change, e.g. very high noise level, because an increase in the ambient noise above the starting point of the expansion causes the noise to be transmitted with increased amplification.
In such electro-acoustic installations, for example a telephone with intercom, the acoustic anti-sidetone depends on the arrangement of the directional effect of the sound transducer in the device, on the sensitivity of the microphone being used, and the adjusted reproduction volume. With the known compander method, a curve displacement is possible as a function of incoming signal. The expander range can be displaced in such a way, that the signal emitted by the loudspeaker and received by the microphone is always securely suppressed when a corresponding adjustment has been made on the terminal, i.e. by the manufacturer.
The use of such known, standardized circuits for terminals of different construction, e.g. with different anti-sidetones through both the housing and the air path, loudspeaker-microphone and/or the use of loudspeakers and/or microphones with different efficiencies, or when they are used in spaces that are acoustically very different from each other, it can happen that the possible manual adjustment of the anti-sidetone is no longer sufficient to ensure good vocal communication during use of the intercom.
For that reason, it was already proposed (in the applicant's P 42 29 910) to additionally control the compander as a function of the outgoing and incoming signal, by determining and weighting the voice level of the two signals, and utilizing the comparison value with the amplitude of the incoming signal to control the curve. The result is an anti-sidetone that adjusts to the ambient and terminal conditions, therefore an adaptive anti-sidetone. Thus, no manual adjustment and adaptation are required any more. This circuit functions as automatic echo damping with respect to the anti-sidetone, hereafter called AERL.
With unfavorable microphone-loudspeaker arrangements, particularly in small terminals, the intercom quality is not assured in all instances, even with AERL-control.